Call for agenda for Workstation WG meeting 2014-Dec-17

[Chris Murphy]

On Tue, Dec 16, 2014 at 12:21 PM, Josh Boyer <jwboyer at fedoraproject.org> wrote:
> Your requests aren't bad at all, but they aren't new.  Everyone wants
> btrfs because it was hyped as the filesystem of the future.

tl;dr Those who want it, want it for the features, and much simpler
access to those features. Not because of hype.

Raid, resizing, snapshots, are much easier with Btrfs separately let
alone in combination. And they're safer because the nomenclature is
nowhere near as obscure or varied due to the differing syntax of
mdadm, lvm, and mkfs. Today.

mkfs.btrfs -draid5 /dev/sd[bcde]

vs

mdadm -C md0 -n 4 -l raid5 /dev/sd[bcde]
mdadm --detail --scan >> /etc/mdadm.conf
pvcreate /dev/md/md0
vgcreate VG /dev/md/md0
lvcreate -l 95%FREE -T VG/thinp -V 2500M -n home
mkfs.xfs /dev/VG/home

mkfs.btrfs doesn't need to sync, so it's a < 2 second mkfs. It's hours
or days for conventional raid sync.

How about growing by adding a new device?

btrfs device add /dev/sdf <mountpoint>

vs

mdadm --add /dev/md/md0 /dev/sdf
mdadm --grow --raid-devices=5 /dev/md/md0
pvresize /dev/md/md0
lvextend -L+100G VG/home
xfs_growfs /dev/VG/home

The mdadm add/grow requires a reshape. Btrfs doesn't. And there are
inefficiencies in resizing XFS or ext4 that don't apply to Btrfs.
Today, there's some bug preventing the grow/reshape from completing,
so it's not like everyone except Btrfs is immune to showstopper bugs.
So right now I can't even complete the above grow for md+lvm+xfs,
while the raid5 one not only works but it's faster.
https://bugzilla.kernel.org/show_bug.cgi?id=89851

How about I take a snapshot of /home, and then accidentally delete a
file and want to rollback?

btrfs subvolume snapshot /home /home.1
rm -f /home/chris/importantdonteverdelete.tar.gz
cp --reflink /home.1/chris/importantdonteverdelete.tar.gz /home/chris/

This completes instantly regardless of the file's size.

vs

lvcreate -s --name home_snap VG/home
rm -f /home/chris/importantdonteverdelete.tar.gz
lvchange -ay -K VG/home_snap
mount -o nouuid /dev/VG/home_snap /mnt/homesnap
cp /mnt/homesnap/importantdonteverdelete.tar.gz /home/chris/

Not the end of the world, but it's still more lines to remember and
type, I had to man lvchange to recall the -K, and this method requires
a full file copy which is slower and takes up space in the pool.

-- 
Chris Murphy